Why can't I update my iPhone using unlimited data?

Updates can only be performed over Wi-Fi or 5G. Your iPhone XR does not have 5G capabilities.

It is the carriers that do not want large data transfers such as software updates to occur over slower networks. They have final say over what "unlimited data" actually means.

Any iOS update that your phone downloads will have a checksum to protect against the chance of the file being corrupted by errors in data transmission. It might also have a cryptographic signature to assure that it is from Apple. Barring a rather sophisticated attack that is unlikely to happen at a McDonald’s, your phone would connect to Apple’s servers whether or not the subject of the conversation (iOS 18) could be eavesdropped upon.

In short, your claims that it is “dangerous” to update using a McDonald’s Wu-Fi hotspot are vastly overblown. It is far more likely that the other users at McDonald’s, or the restaurant itself, would have an issue with you, for sucking down so much of their bandwidth.

5G was still years in the future when your iPhone XR was designed. Even if you buy a new phone now, it won't be able to handle 6G or whatever it will be called in another 4-5 years.

Make sure that your next phone is 5G capable. That would mean an iPhone 12 or higher model.

By the way, not all carriers will allow updates over 5G.....even if the phone supports 5G....so you would want to check with your carrier about this as well when you are looking for a new phone.

There is no way your phone is using 5G, it simply doesn't have the hardware required for those frequencies.

If you have AT&T as your carrier, it might say 5Ge which is not really 5G, it's false advertizing on AT&T for their modified 4G signal.

Speed can be hindered by many things, interference from motors, florescent lighting, anything that causes emf, etc. Also delayed signal reflections off of buildings, and a myriad of other influences.

Also cell tower transmitters can only handle so much bandwidth. If hundreds/thousands of cell phones in a big city are using a transmitter, that bandwidth is shared by all those phones. In a rural area, there may be only a few phones sharing the bandwidth of the transmitter.

It actually makes a lot of sense having a faster speed in a rural area as long as the signal strength is good.

But when I go visit a friend in a big city my 4g lte changes to saying 5g

No, it doesn't. Look more carefully and you will see something like 5Ge or 5 GE......which is NOT 5G.

Even if really was 5G, your iPhone XR could not support 5G

Your phone needs to support 5G, where you can get software updates over cellular. I suppose bandwidth is a factor for the demand placed on the cellular provider for the slower network speeds.